Population Abundance and Body Size of Calanus Sinicus in Marginal Habitats in the Coastal Seas of South-Eastern Hong Kong G.-T
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Journal of the Marine Biological Association of the United Kingdom, 2013, 93(1), 135–142. # Marine Biological Association of the United Kingdom, 2012 doi:10.1017/S0025315412000938 Population abundance and body size of Calanus sinicus in marginal habitats in the coastal seas of south-eastern Hong Kong g.-t. zhang1 and c.k. wong2 1Jiaozhou Bay Marine Ecosystem Research Station, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China, 2School of Life Sciences, The Chinese University of Hong Kong, Shatin, Hong Kong, China The species range of Calanus sinicus along the Chinese coast extends from the Bohai Sea and the Yellow Sea in the north to the northern part of the South China Sea in the south. The subtropical seas along the southern coast of China mark the southern edge of the range of C. sinicus. In coastal seas off eastern Hong Kong, C. sinicus appears first in December, but densities com- parable to those in the Yellow Sea and the East China Sea are reached only in January and February when temperature is ,158C. Density decreases in March as temperature increases. No individuals remain after May when temperature is .258C. The average prosome length of females collected in February is comparable to that of females from the Yellow Sea, but females collected after mid-March are smaller than the smallest females from the Yellow Sea. Reproduction occurred mainly between January and March. Rapid decline of the population in April and the absence of a summer population suggest that the local population is derived from individuals advected from the north by ocean currents. Eggs produced locally probably did not hatch or develop into adults. Keywords: Calanus sinicus, population abundance, body size, subtropical coastal waters Submitted 22 May 2012; accepted 5 June 2012; first published online 4 September 2012 INTRODUCTION season from April to September. Oceanic conditions, charac- terized by high salinities, prevail on the eastern side of Hong Calanus sinicus is a calanoid copepod with wide distribution Kong. The eastern side of Hong Kong consists of two semi- in continental shelf waters of the north-west Pacific Ocean enclosed bays in the north and open sea area in the south (Hulsemann, 1994). Along the Chinese coast, the range of (Figure 1). Tolo Harbour, a landlocked bay opens into Mirs C. sinicus extends from the Bohai Sea and the Yellow Sea in Bay via a narrow tidal channel. Mean water depth in Tolo the north to the northern part of the South China Sea in the Harbour varies from 10 m in the inner shallow basin to south (Chen, 1992). In the temperate waters of the Bohai about 20 m in the channel. Because of the semi-enclosed topo- Sea, the Yellow Sea and the East China Sea, C. sinicus graphy, Tolo Harbour is characterized by low current velocity occurs throughout the year and is a dominant member of and is poorly flushed (Oakley & Cripps, 1972). Bordered by the mesozooplankton in terms of numerical abundance and Hong Kong on the west and mainland China on the north ecological importance (Chen, 1992; Wang et al., 2003). In sub- and east, Mirs Bay has an average water depth of about tropical seas along the coast of southern China, C. sinicus 25 m and is fully exposed to water currents from the South occurs seasonally from late autumn to early spring (Chen, China Sea. 1992; Hwang & Wong, 2005). While C. sinicus is considered Results of a previous study revealed that the occurrence of one of the key species of planktonic copepods in the coastal Calanus sinicus in coastal oceans around Hong Kong and waters of China and Japan (Chen, 1964), most previous Taiwan is limited by temperature to winter and spring studies on the seasonal dynamics and life history of C. (Hwang & Wong, 2005). It was hypothesized that C. sinicus sinicus have focused on populations in temperate oceans is carried into the subtropical oceans in the southern part of (Uye, 2000; Zhang et al., 2005; Xu & Chen, 2007; Zhang the species range every winter from population centres in et al., 2007). Far less information is available on populations the north by the China Coastal Current when the north- in the subtropical waters near the southern edge of the eastern monsoon is blowing (Chen, 1992). Understanding species range. the life history of C. sinicus in marginal habitats near the Hong Kong is located in the southern part of the Chinese southern edge of its geographical range is important to coast. A plume of estuarine water from the Zhujiang River researchers of the ecosystem dynamics of the South China covers the western side of Hong Kong during the rainy Sea. In this study, seasonal variations in population structure and body size of C. sinicus are investigated along a transect extending from the inner part of Tolo Harbour to the outer Corresponding author: part of Mirs Bay. The objectives are to: (i) investigate the C.K. Wong population structure of C. sinicus in subtropical coastal Email: [email protected] oceans; (ii) identify factors that influence the seasonal and 135 136 g.-t. zhang and c.k. wong Fig. 1. Map of Hong Kong showing the locations of Tolo Harbour, Mirs Bay and the six sampling stations. spatial distribution patterns of C. sinicus; and (iii) provide pre- by hauling a conical plankton net (0.5 m mouth diameter; liminary information on the recruitment mechanisms of C. 125 mm mesh) vertically from 2 m above bottom to the sinicus in marginal habitats in the northern part of the surface. All zooplankton samples were immediately preserved South China Sea. in 4% formalin seawater. Water temperature and salinity were measured at the surface (0.5 m) using a Hydrolab H20 water quality monitoring system (Austin, USA). Chlorophyll-a con- MATERIALS AND METHODS centration in seawater samples collected at the surface (0.5 m) with a Van Dorn type bottle was measured fluorometrically Zooplankton was collected approximately semi-monthly from using a Turner Designs 10-AU-000 Fluorometer (Sunnyvale, September 2003 to July 2004 at 6 sampling stations. Two USA) (Strickland & Parsons, 1968). stations, S1 (22826′725′′N, 114814′566′′E) and S2 In the laboratory, C. sinicus in different developmental stages (22828′539′′N, 114818′034′′E) were located inside Tolo (eggs, nauplii (NI–NVI), copepodites (CI–CV), adult males Harbour. Two stations, S3 (22829′468′′N, 114821′588′′E) and and adult females) were identified and sorted under a stereomi- S4 (22826′670′′N, 114826′920′′E) were located in Mirs croscope according to the descriptions of Li & Fang (1983). Bay. Two stations, S5 (22817′560′′N, 114826′920′′E) and S6 Densities were determined by counting 25–100% of each (22813′000′′N, 114826′920′′E) were located in the coastal sea sample under a stereomicroscope. Prosome lengths, from the of south-eastern Hong Kong outside Mirs Bay (Figure 1). anterior end of the cephalosome to the posterior lateral edge Water depths ranged from 15 m at S1, the innermost of the last metasome segment, of CVs, adult males and adult station, to 30 m at S5 and S6, the two outermost stations. females were measured. At least 10 individuals of each develop- At each station, duplicate zooplankton samples were taken mental stage were measured for each sample. RESULTS Environmental conditions Similar seasonal trends in temperature and salinity were observed at all sampling stations (Figure 2). Surface water temperature ranged from 158C in late January and early February to .258C in May. Salinity ranged from a maximum of 37 at S2 in February to a minimum of 31 at S1 and S3 in early May (Figure 2). Salinity decreased markedly after March, especially in Tolo Harbour, with the approach of the rainy season. Chlorophyll-a concentration was much higher in Tolo Harbour than in Mirs Bay (Figure 3). In Tolo Harbour, values spanned widely from 2.67 to 20.30 mgl21 at S1 and m 21 Fig. 2. Temporal variations in water temperature and salinity in the study area from 0.84 to 8.70 gl at S2. In comparison, values recorded during the period when Calanus sinicus was present. Values represent average at the four stations outside Tolo Harbour varied from 0.17 to 21 of six sampling stations. Error bars denote the range. 4.36 mgl . calanus sinicus in the coastal seas of south-eastern hong kong 137 in April and May. Eggs of C. sinicus appeared first in samples collected in December but did not become abundant until late January. The maximum average density of 485 ind. m23 was recorded in early February. No eggs of C. sinicus were found in samples collected in May. Calanus sinicus exhibited spatial variations in abundance throughout the study period (Figure 4). Adults, copepodites, nauplii and eggs appeared at S5 and S6, the outermost stations in December, but did not reach Tolo Harbour until January. Adults and copepodites appeared at S5 and S6 in December, reached S3 and S4 in Mirs Bay in early January, and were found in Tolo Harbour at the end of January. Throughout January and February, the density of adults was much lower at S1 and S2 than at the four outer stations. A similar trend of shoreward decrease was observed in the density of nauplii and copepodites in January. Samples collected in Tolo Harbour in early January contained no nauplii. A few eggs were found in S2 in early January when the average density of eggs in the entire study area was still ,50 m23, but samples collected in Tolo Harbour in late January did not contain eggs even though egg density at S6, the outermost station, had reached a maximum of .1400 m23.